In IS-95 technology, a Rake receiver has enhanced a SNR (signal-to-noise ratio) performance by combining multi-path signals. But, each of the multi-path signals has a time delay, so that a timing synchronization among the plurality of multi-path signals should be regulated so as to combine the multi-path signals. Therefore, each finger of a Rake receiver should have an independent FIFO (First-In First-Out) register in order to achieve a timing synchronization of the multi-path signals. Such a structure does not generate a serious problem with respect to IS-95 technology because the number of FIFO registers is small and the size of FIFO registers is small as well.
However, with the trend of evolving of a mobile communication system to IMT-2000 system, a high-speed data transmission is needed. As the number of fingers increases and a symbol duration is reduced, the number of FIFO registers and the size should be increased abruptly. In IMT-200 system where the hardware complexity factor of the Rake receiver becomes a very important problem, abruptly increasing the number of the hardware of the FIFO registers causes a very serious problem in designing the Rake receiver. Therefore, there is a limitation in designing the Rake receiver of IMT-2000 system by using a conventional symbol combining algorithm.
FIG. 1 is a conceptual diagram illustrating a symbol combining method of a conventional IS-95 system.
Each finger has its own FIFO registers, demodulates a symbol in response to its own demodulation time, and stores the demodulated symbols (1), (2) and (3) in the blocks (1), (2) and (3) of a FIFO register of each finger. If the symbols are accumulated, a combiner reads a plurality of symbols (4),(5) and (6) having the same timing reference in each finger and then combines them (4)–(6). In the meantime, a depth of FIFO register of FIG. 1 should have a magnitude by which a signal is not lost in a hand-off state or a multi-path fading state. In a conventional IS-95 system, the depth of the FIFO register has been designed in eight-stages.
As shown in FIG. 1, in the conventional symbol combining method, each finger should employ its own FIFO register so as to adjust a timing synchronization of the demodulated symbols. This is not considered as an important problem in case of a system like IS-95 system having a small number of fingers and a shallow depth of FIFO, but this poses a very considerable problem in IMT-2000 system having a great number of fingers and a large-sized FIFO.
FIG. 2 is a table illustrating the increase of a hardware complexity of FIFO register in case that a conventional symbol combining algorithm is applied to the IMT-2000 system as it is.
Referring to FIG. 2, a first increase factor (1) corresponds to the increasing degree of FIFO register's depth in IMT-200 system as compared with IS-95 system. A second increase factor (2), a third increase factor (3), and a fourth increase factor (4) correspond to the increasing degree of the number of FIFO registers in IMT-2000 system as compared with IS-95 system. Considering all of the increase factors, it can be seen that the FIFO register of the IMT-2000 system has a hardware complexity 192 times greater than that of the FIFO register of the IS-95 system. This result means that a conventional symbol combining algorithm may bring about a very serious problem in the IMT-200 system, in which the hardware complexity is considered as the most important factor.